This invention relates to polytetrafluoroethylene (PTFE) and particularly to shaped compositions of PTFE having substantially improved resistance to cold flow deformation under load (creep) and a novel process for their fabrication.
PTFE is a well-known and highly useful polymeric material typically employed in demanding applications involving exposures to high temperatures and/or highly corrosive environments. However PTFE also exhibits cold-flow or creep to a degree considered excessive in many applications. High creep results in increased cost to the user in the form of maintenance, down-time, and unexpected failure. In typical commercial practicer creep is reduced by incorporating fillers into PTFE by dry mixing prior to forming a shaped article. Fillers may include minerals, graphite, glass and polymeric materials. Because of the wide range of chemically hostile environments in which PTFE is used, it is frequently found that a filled PTFE composition suitable for one particular application is unsuitable for other applications. Thus, specific formulation may be required for each application. Moreover, many fillers increase abrasiveness and reduce wear resistance. A common remedy for high creep in PTFE is to "design around" it, i.e., to devise, sometimes costly, engineering solutions to compensate for the expected creep of the polymer.
Commercially available PTFE is a thermoplastic polymer of unusually high molecular weight which cannot be fabricated using conventional techniques such as melt extrusion or injection molding. Instead, powder processing methods are typically applied to form shaped articles from PTFE. These methods include direct forming of shaped articles by cold compaction of PTFE powder followed by "free sintering", ram extrusion, paste extrusion, and high isostatic pressure processing. Most commonly, billets are formed by cold compaction and free sintering, followed by machining into the final shaped article. In "cold compaction" resin is placed in a mold and formed into a fragile, partially consolidated shaped article by the application of pressure, typically 20-40 MPa, typically at room temperature, but, in any event, at least 50.degree. C. below the melting point of the resin. In "free sintering" the object produced in the "cold compaction" step is removed from the mold and placed unconstrained in an oven where consolidation is completed by the application of temperatures at or above the melting point.
Typical commercial processing of PTFE results in shaped articles which creep about 2.5% as measured by ASTM D-621 method A (Deformation under load). Thomas et al. (Soc. Plastics Engrs., 12, 505 (1956)) report creep as low as 1.5% but the method by which this result is achieved is not made clear in the Thomas et al. text and the result does not appear to be reproducible within that text.
Chemical Abstract CA 113(22):192949n (Hungarian Application HU 51542 A2, published May 28, 1990) describes an apparatus and method for compression molding PTFE wherein 3-10 mm thick PTFE plates fastened to a metal center piece are prepared by rapidly compression molding PTFE powder at 390.degree. C. and 200 kg/cm.sup.2 pressure in an apparatus having a spring-supported base.
The inclusion into PTFE of additives other than fillers, such as lubricants, plasticizers and processing aids, is widely practised. The additives are usually solids or high boiling liquids having some compatibility with PTFE at least at elevated temperatures. U.S. Pat. Nos. 4,360,488 and 4,385,026 disclose formation of "non-draining" gels by heating PTFE with a highly fluorinated low molecular weight material at a temperature close to the crystalline melting point of the polymer (330.degree.-350.degree. C.). A solution or swollen mass containing from about 1 to about 50 weight % polymer is formed on heating from which is recovered on cooling a sponge-like gel, said gel retaining no "memory" of the crystallinity of the original PTFE. The gel, after removal of the fluorinated material by extraction in refluxing solvent such as FC-113 (bp 45.8.degree. C.), was described as porous and could be formed into porous shapes, e.g., into porous sheet by pressing between platens. The porous products had increased crystallinity and a partially fibrillar structure. Use as filter membranes or diaphragms for electrochemical cells was disclosed.
U.S. Pat. No. 4,110,392 describes preparation of porous PTFE products wherein PTFE powder or coagulated PTFE dispersion is blended with a liquid lubricant which can include fluorocarbon oils, and the blend is shaped by extrusion and/or rolling into sheet, film or tubing. Shaping is carried out below about 327.degree. C., usually at room temperature. The unsintered shaped article is made porous by stretching in at least one direction, before or, preferably, after, removal of the lubricant. Lubricant-free articles are then sintered at above 327.degree., commonly about 360.degree. C., and re-stretched below 327.degree. C. to develop fine pores of 100 to 1,000 A diameter.